The impact of droughts and water management on various hydrological systems in the headwaters of the Tagus River (central Spain)

The influence of climate variation on the availability of water resources was analyzed in the headwaters of the Tagus River basin using two drought indices, the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI). This basin is highly regulated and strategic, and contains two hyperannual reservoirs that are the origin of the water supply system for Mediterranean areas of southeast Spain. The indices confirmed that drought conditions have prevailed in the headwaters of the Tagus River since the 1970s. The responses in river discharge and reservoir storage were slightly higher when based on the SPEI rather than the SPI, which indicates that although precipitation had a major role in explaining temporal variability in the analyzed parameters, the influence of temperature was not negligible. Moreover, the greatest response in hydrological variables was evident over longer timescales of the climatic drought indices. Although the effect of climate variability on water resources was substantial during the analyzed period, we also showed a major change in hydrological–climatic relationships in regulated systems including reservoir storage and outflow. These were closely related to changes in external demand following commencement of the water transfer system to the Júcar and Segura basins after the 1980s. The marked reduction in water availability in the basin, which is related to more frequent droughts, contrasts with the amount of water transferred, which shows a clear upward trend associated with increasing water demand in the Mediterranean basin.